Slurry is handled in various scenes in chemical industries. Some problems occur on handling slurry due to solid contained in slurry. One of the problems is that it is necessary that slurry is flowed consistently for preventing solid contained in the slurry from being sedimented in a vessel. Accordingly, slurry is generally housed in an agitation vessel having an agitator.
In the case where crystals are formed and/or grown in slurry, however, i.e., a liquid phase constituting the slurry has dissolving power to crystals in the slurry, such a phenomenon frequently occurs that crystals are accumulated on the wall of the agitation vessel and the agitation blade. This is because not only crystals in the slurry are sedimented, but also crystals newly deposited from the liquid phase are bound with the crystals thus sedimented and then agglomerated to form large-sized solid matters.
The accumulated matters may be released from the wall of the vessel due to impact or the like and may be mixed into the slurry. The released accumulated matters are generally not broken into the size of the crystals in the slurry, and are often present therein as an agglomerate having been released as it is or a pulverized matter thereof.
Extraction of slurry is generally carried out through an extraction tube provided at a bottom of the vessel for preventing dead space from occurring. The open end of the extraction tube is generally at the same position as the bottom of the vessel. The agglomerate or pulverized matter thereof may flow into the extraction tube to cause flowage failure or to cause complete obstruction. Various kinds of agitation devices have been proposed, but it is technically difficult to obtain a completely mixed state of slurry by agitation. In particular, such a phenomenon cannot be avoided that solid contained in the slurry is locally sedimented, and the solid may flow into the extraction tube to cause flowage failure or obstruction of the extraction tube. The flow rate is quickly increased at the part where the slurry enters the extraction tube from the vessel. Accordingly, a solid matter having a large weight remains at the extraction part where the change in flow rate becomes maximum, whereby the solid matter may form a bridge at the inlet of the extraction tube to cause flowage failure or obstruction of the extraction tube.
For example, in a process of producing terephthalic acid through hydrolysis of dimethyl terephthalate in an agitation vessel, the most part of terephthalic acid produced is suspended as crystals in an aqueous solution since the solubility of terephthalic acid in the aqueous solution is relatively small under the reaction condition. A part of the crystals is accumulated on the wall of the reaction vessel or an agitator to form a stiff solid matter. When the solid matter is released due to impact or the like and mixed into the slurry, the solid matter may flow into the extraction tube at the bottom of the vessel to fail to send the slurry due to obstruction of the extraction tube.
In a process of continuously sending slurry from an agitation vessel to another agitation vessel under a lower pressure through a decompression valve, the obstruction is liable to occur since the decompression valve has a part having a narrower flow path than the tube.
In order to solve the problem, Patent Document 1 proposes such a measure that an open end of an extraction tube, which is provided at a bottom of a vessel, is provided to protrude from the bottom by 50 mm or more. However, such an agitator is also proposed that cannot have an open end protruding from the bottom by 50 mm or more, depending on the configuration of the agitator, and the application range thereof is limited.
(Patent Document 1) JP-A-8-141386